diff --git a/mkosi.build b/mkosi.build
index 0e644d54cb..7cdcd6c741 100755
--- a/mkosi.build
+++ b/mkosi.build
@@ -25,7 +25,28 @@ set -ex
 # as out-of-tree build dir. Otherwise, let's make up our own builddir.
 [ -z "$BUILDDIR" ] && BUILDDIR=build
 
-export LC_CTYPE=en_US.UTF-8
+# Meson uses Python 3 and requires a locale with an UTF-8 character map.
+# Not running under UTF-8 makes the `ninja test` step break with a CodecError.
+# So let's ensure we're running under UTF-8.
+#
+# If our current locale already is UTF-8, then we don't need to do anything:
+if [ "$(locale charmap)" != "UTF-8" ] ; then
+        # Try using C.UTF-8 locale, if available. This locale is not shipped
+        # by upstream glibc, so it's not available in all distros.
+        # (In particular, it's not available in Arch Linux.)
+        export LC_CTYPE=C.UTF-8
+        if [ "$(locale charmap)" != "UTF-8" ] ; then
+                # Finally, try something like en_US.UTF-8, which should be
+                # available in Arch Linux, but is not present in Debian's
+                # minimal image in our mkosi config.
+                export LC_CTYPE=en_US.UTF-8
+                if [ "$(locale charmap)" != "UTF-8" ] ; then
+                        # If nothing works, fail early.
+                        echo "*** Could not find a valid locale that supports UTF-8. ***" >&2
+                        exit 1
+                fi
+        fi
+fi
 
 if [ ! -f "$BUILDDIR"/build.ninja ] ; then
         sysvinit_path=`realpath /etc/init.d`